With increasing channel transmission rate (in particular in case of 40 Gbps and beyond), PMD severely degrades the signal quality. The reason for PMD is that an optical fiber exhibits two orthogonal principle axes corresponding to two principle states of polarization (PSP), which have different travelling speeds. A signal's first portion aligned with one principle axis propagates with different speed than a signal's second portion aligned with the other principle axis. Therefore, the two portions separate along the fiber, thereby spreading the signal pulses and causing signal interference between subsequent symbols (also called ISI—intersymbol interference). The delay between both portions of the signal is characterized by the differential group delay (DGD), which is proportional to the square root of the travel distance.
A polarization mode dispersion compensator (PMDC) may be used upstream of a receiver to compensate for the fiber-induced PMD. Such PMDC is controlled by a feedback loop. A feedback signal is derived from the optical signal downstream of the PMDC (i.e. after compensation). Such feedback signal gives an indication of the current degree of PMDC compensation. Based on the feedback signal, one or more parameters of the PMDC are adapted to improve the compensation. Various approaches for generating a feedback signal are known: One option is a degree of polarization (DOP) based feedback signal, where the DOP indicates the portion of the optical beam which is polarized. An alternative is a feedback signal based on measuring a part of the RF-spectrum of the carrier modulation. Here, the feedback signal may be based on one or several spectral lines. According to another alternative, it is also possible to monitor the received eye (e.g. the eye opening) and to use this information as an eye monitor feedback signal. Alternatively, an error signal generated by a FEC (forward error correction) algorithm may be used as a feedback signal. The latter two approaches have the drawback that they require a complete receiver with clock and data recovery; in the latter approach also an FEC unit is needed. In case an extra receiver is dedicated to this purpose, the costs are increased. In case no extra receiver is used but the feedback signal is derived from the receiver downstream of the PMDC, the flexibility of the PMDC is reduced since it cannot be used as a stand-alone device anymore.
To enhance tolerance to intra-channel non-linear effects, in particular in case of high data rates (such as 40/43 Gbps—gigabit per second), alternate-polarization (APol) modulation formats may be employed, which use alternate polarization for each symbol, i.e. adjacent symbols have orthogonal polarizations. A very promising modulation format for 40/43 Gbps data rates is Apol RZ DPSK (return-to-zero differential-phase-shift-keying).
Due to the alternate polarizations in APol modulation formats, the generation of a feedback signal for a PMDC in a transmission system using APol modulation is more difficult.
For Apol modulation, using DOP as feedback is not suitable since an Apol signal is inherently depolarized.
Generating a feedback signal by monitoring the intensity of a radio-frequency (RF) spectral line is also problematic in case of Apol modulation. When measuring the RF-tone at half the symbol rate (e.g. at 20/21.5 GHz in case of 40/43 Gbps Apol RZ DPSK), such feedback signal is “blind” when the alternate-polarization pulses of the optical are launched with a 45° polarization offset to the principle axes of the fiber, i.e. one polarization of the two alternate polarizations has a 45° offset compared to one of the principle axes of the fiber. For a 45° launch, the feedback signal does not change in dependency on the remaining DGD (i.e. the non-compensated DGD) and thus does not provide any information about the current degree of compensation.
A solution to overcome this problem when monitoring the intensity of a RF spectral line is to use a fast polarization scrambler (typically at the transmitter) as discussed in the application PCT/FR2008/051865 having the title “Apparatus and method for compensating polarization mode dispersion” and filed on Oct. 15, 2008 Such polarization scrambler varies the splitting ratio among the principle axes such that a mean feedback signal over all polarization states is generated (i.e. for all launching polarization offsets). Typically, scrambler frequencies considerably higher than 1 MHz are used.